In addition, there have been recent reports of an alliance - and possibly, down the road, a merger - of Cosco and China Shipping Container Lines. Trade observers view that development as a direct response to the coming of the P3 and enhanced G6.

All of these moves are being driven by one motive: the intense desire of carriers to cut operating costs. "This is just Economics 101," said Adam D. Hall, senior director of international logistics with Dollar General Corp., at the TPM conference. Carriers are sharing resources to address a scarcity of available freight, he said.

Both the P3 and expanded G6 agreements are subject to approval by the Federal Maritime Commission, but that body has never rejected a VSA or sought an injunction due to competitive concerns. (FMC's current review period for the P3 ends on March 24, and for the G6 expansion on April 4.) In addition, the European Commission's Directorate-General for Competition reserves the right to challenge any agreements that touch its members' shores, but the services can go into effect in the meantime.

With the agreements comes a flood of ever-larger containerships. Assuming it passes regulatory muster, the G6 will offer an average per-vessel capacity of just under 11,500 twenty-foot equivalent units (TEUs), slightly more than the CKYHE, said Lars Jensen, chief executive officer and partner with SeaIntel Maritime Analysis. But it's the P3, averaging 13,000 TEUs per ship, that will be the real game-changer. Pushing up that number are Maersk's Triple-E class vessels, with record capacity of up to 18,000 TEUs.

"There is no way on a unit-cost basis that others can be competitive with the P3," said Jensen. "It's just not feasible."

The largest ships will mostly see service in the Asia-Europe trades, but their impact will be felt worldwide. Vessels currently sailing on those routes, ranging between 8,000 and 10,000 TEUs, will be "cascaded" into the trans-Pacific, Europe-South America and Asia-Middle East trades, bumping up capacity levels everywhere.

Absent a proportionate rise in demand, the capacity boost could cause extreme rate volatility in certain trades. Service integrity will be affected as well, said Jensen. Carriers are increasingly using the cancellation of sailings as a tool for keeping rates high. Expect that practice to become even more common.

The combination of mega-alliances and megaships is creating a market that's based entirely on low rates and high operating efficiency. Like it or not, said Jensen, carriers are fashioning a commoditized, "cookie-cutter" service. As a result, alliance partners must find other ways to differentiate their product, whether through better intermodal connections, document handling or customer service.

Ultimately, carriers are hoping to offer a combined offering with so many sailings that shippers won't notice the cancellation of a particular voyage. "It's no different than if you send a package with FedEx," Jensen said. "You don't worry about which plane your cargo is on."

That's all well and good, but the alliances will also have the effect of reducing shippers' options. And the size and scale of the carrier networks will make it impossible for newcomers to enter the commoditized trades. Said Jensen: "The gate will be shut."

Will rates become more or less stable as a result? On one hand, shippers will get increased transparency in pricing, which reduces rate volatility. On the other, alliance members could engage in a bloodbath of discounting as they compete for a shipper's business. "You could argue either way," said Jensen.

Bill Woodhour, vice president of North American trades with Maersk Line, said the alliances are a response to a slowdown in trade growth, a deflationary rate environment and the glut of new ship capacity that will enter service in the coming years. "The best proposition for us and our partners was to work together," he said.

Shippers are on the fence. Klaus Schnede, manager of marine, air and facilities procurement with Eastman Chemical Co., said his company initially welcomed the announcement of the P3 and G6 combinations. "We saw it as a natural evolution - the next step in the industry to foster more cost savings, become more efficient and provide better service."

At the same time, Eastman is concerned that it might book with multiple carriers, only to see its cargoes end up on the same ship. That scenario makes the company's supply chain more vulnerable to disruption. For now, it views the alliances as both positive and negative, "but the positives are greater," said Schnede.

Hall worries that the alliances will affect service reliability. Carriers' on-time record of 64 percent in the fourth quarter of 2013 was one of the worst he's seen in the trans-Pacific trades. Any delayed or canceled sailings could seriously impact Dollar General's seasonal sales, he said.

Hall further wondered whether carriers can shift cargo from one sailing to another as easily as Jensen suggested. P3 alliance members hope to allay those concerns through creation of a network operations center that would coordinate bookings secured by the individual lines.

Will they succeed? Shippers will be watching closely for the answer. So, one hopes, will the FMC and European Commission regulators.